1. Field of the Invention
The present invention relates to a method and apparatus for treating a disc plate. More specifically, the present invention relates to the method and the apparatus for reducing a warp of an optical disc such as a digital video disc (hereinafter referred to as “DVD”) and a compact disc. Especially, the warp is the one that is caused to a disc plate (of the optical disc) which has been molded through an injection molding. With the thus reduced warp, the disc plate is flattened.
2. Description of the Related Art
Generally, an optical disc plate such as a DVD plate and a compact disc plate is molded through an injection molding.
As is seen in FIG. 14(A), FIG. 14(B) and FIG. 14(C), there is provided a method of molding a disc plate 1. A resin material is injected into a pair of a first metal mold 10 and a second metal mold 11 for an injection molding. After the injection molding, the resin material is molded into a mold disc plate 1′ which is to be solidified to become the disc plate 1. At this point in time, a predetermined information is recorded on one of two surfaces of the mold disc plate 1′. After this, as is seen in FIG. 14(B), the first metal mold 10 and the second metal mold 11 are separated from each other (opened). Then, a takeout arm 12 of a takeout mechanism (not shown) moves forward into an area between the first metal mold 10 and the second metal mold 11, to thereby securely absorb the mold disc plate 1′ which is high in temperature and is still soft. After this, the takeout arm 12 moves backward, to thereby take the mold disc plate 1′ out of the first metal mold 10 and the second metal mold 11.
Then, as is seen in FIG. 14(C), in a predetermined position, the thus taken out mold disc plate 1′ is fixedly absorbed directly with a conveyer arm 13 of a conveyance mechanism. The conveyer arm 13 conveys the mold disc plate 1′ to a cooling stage 14 (next operation).
More specifically, the mold disc plate 1′ has a first surface and a second surface opposite to the first surface. On the first surface, the mold disc plate 1′ is fixedly absorbed to a tip end 12A of the takeout arm 12. On the second surface, the mold disc plate 1′ is fixedly absorbed to a tip end 13A of the conveyer arm 13. Simultaneously with the absorption of the mold disc plate 1′ to the tip end 13A, the tip end 12A releases (namely, stops absorbing) the mold disc plate 1′. Then, the conveyer arm 13 turns by substantially 180° with the mold disc plate 1′ fixedly absorbed to the tip end 13A. Thereafter, the conveyer arm 13 conveys the mold disc plate 1′ to the cooling stage 14. The mold disc plate 1′ is cooled with the cooling stage 14, to thereby solidify the resin material of the mold disc plate 1′. After the solidification, the mold disc plate 1′ is referred to as the disc plate 1. The solidification temperature for solidifying the resin material is, for instance, about 90° C.
As described above, the mold disc plate 1′ soon after being taken out of the first metal mold 10 and the second metal mold 11 is high in temperature, and is sufficiently soft. Therefore, during the sequential operations of i) cooling, ii) solidification of resin material, and iii) becoming the disc plate 1, the mold disc plate 1′ causes a considerable deformation such as warp. Conventionally, various methods are taken so as to reduce the warp by adjusting conditions of the injection mold as below:    i) Adjust temperature of the first metal mold 10 and the second metal mold 11 of a mold machine.    ii) Set up temperature difference between the first metal mold 10 and the second metal mold 11.    iii) Adjust pressure for injecting the resin material.
Even if the warp of the mold disc plate 1′ is small and reduced through the conventional methods as described above, the mold disc plate 1′ causes the warp during the time the mold disc plate 1′ is taken out of the first metal mold 10 and the second metal mold 11, and during the time the mold disc plate 1′ is conveyed to the cooling stage 14. The thus caused warp is attributable to the following repeated description, that is, “The mold disc plate 1′ soon after being taken out of the first metal mold 10 and the second metal mold 11 of the mold machine is high in temperature and is sufficiently soft.” Moreover, the thus caused warp grows during the cooling operation.
The warp is more specifically described in FIG. 15(A), FIG. 15(B) and FIG. 15(C). The mold disc plate 1′ is taken out of the first metal mold 10 and the second metal mold 11 of the mold machine. As is seen in FIG. 15(A), the mold disc plate 1′ is fixedly absorbed to the tip end 12A of the takeout arm 12. Moreover (otherwise), the mold disc plate 1′ is fixedly absorbed to the tip end 13A of the conveyer arm 13. The mold disc plate 1′ is high in temperature and is sufficiently soft. Therefore, the mold disc plate 1′ causes the warp when the mold disc plate 1′ is absorbed substantially in the center area of the mold disc plate 1′, in such a manner that the warp is bent opposite {(rightward in FIG. 15(A)} to an absorption side. Especially, the DVD having a thickness of about 0.6 mm (thin) shows the warp (bend) more outstanding than the compact disc having a thickness of about 1.2 mm. Moreover, as is seen in FIG. 15(B), the mold disc plate 1′ is turned by the takeout arm 12 at a comparatively high speed with the mold disc plate 1′ absorbed by the takeout arm 12, and the mold disc plate 1′ is conveyed by the conveyer arm 13 at a comparatively high speed with the mold disc plate 1′ absorbed by the conveyer arm 13. The above turn and conveyance of the mold disc plate 1′ causes a wind pressure, to thereby warp the mold disc plate 1′. Moreover, as is seen in FIG. 15(C), the mold disc plate 1′ is cooled on a disc bench 15 of the cooling stage 14. The mold disc plate 1′ has such a dead weight as to, as the case may be, cause the warp during a cooling time on the disc bench 15. Still furthermore, some other cooling stages having different constitution cause a partial deformation of the mold disc plate 1′ during the cooling time.
The above warp is caused around a center hole (not shown) of the mold disc plate 1′ radially outward, to thereby form substantially a point symmetry. Some other warps run in such a manner as to form a bilateral symmetry (rightward and leftward) with respect to one to three line(s) through the center hole. The above warps become more obvious as production cycle time of the disc plate 1 is shortened, to thereby encourage the warp (more complicated and larger) of the disc plate 1.